Node - Need to reopen texture so it is visible in render mode
When you have opened a texture into a material, then want to modify it by painting on it while it is wrapped around the object, the changes aren't really permanent. In order for the changes to be saved permanently, and this is what I have done, is go into the UV editor, find the texture if it isn't displayed immediately, make sure the changes are visible there, then click on the menu entry Image and either Save As Image if you want it as an external resource, or Pack Image if you want these changes to follow this particular Blend project only.

Hope this was helpful.